Registry

[dalonsoa]

This PR starts the refactoring of the code in solar_cell_solver.py such that it uses registered functions within a set of registries to execute different calculations rather than hardcoding the functionality.

Among other things, using registries enables:

• Checking externally what options are available (eg. by tools like sunglass)
• Expanding the existing functionality with new one without editing Solcore's source code.
• Overwriting existing functionality with custom versions of it to try alternative implementations, again without modifying the source code.

This PR concerns the implementation of the top level ACTIONS of the solar cell solver. PRs for the optical, IV, QE, etc. solvers will follow.

There seems to be lot's of changes because the solar_cell_solver.py file got reformatted by black, but the only important bit in that file is the function solve_solar_cell.

[codecov]

Codecov Report

Merging #231 (1b6fa61) into develop (a5e0dae) will increase coverage by 0.12%.
The diff coverage is 73.91%.

@@             Coverage Diff             @@
##           develop     #231      +/-   ##
===========================================
+ Coverage    45.76%   45.88%   +0.12%     
===========================================
  Files           84       85       +1     
  Lines         9096     9109      +13     
===========================================
+ Hits          4163     4180      +17     
+ Misses        4933     4929       -4     

Impacted Files	Coverage Δ
solcore/solar_cell_solver.py	69.04% <68.42%> (+2.58%)	⬆️
solcore/registries.py	100.00% <100.00%> (ø)

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[phoebe-p]

This is great, I did not know this existed! Following this example, hopefully I can also help converting the rest of Solcore to use registries.